1. Field
The present disclosure relates to the technical field of optical communications, and more particularly to a method for monitoring in-band OSNR (Optical Signal-to-Noise Ratio) based on a parallel asymmetric Mach-Zehnder interferometer.
2. Description of the Related Art
Since the OSNR (Optical Signal-To-Noise-Ratio) is associated with the error rate of the optical signal, the OSNR is one of important diagnostic factors for the health of signal, thus measuring the OSNR is an important diagnostic means for WDM (Wavelength-Division-Multiplexing) systems. Traditional OSNR monitoring on a certain channel is achieved by measuring the level of noise between two channels and performing linear interpolation to determine the noise on the frequency of the channel, i.e., out-band OSNR monitoring. However, with the application of OADMs (optical-add-drop-multiplexer), variant damages may accumulate in the parameters of channels of WDM due to different transmission links, thus causing the out-band OSNR monitoring ineffective. As shown in FIG. 1, FIG. 1(a) illustrates different levels of noise between channels due to different transmission links, and the optical filter or multiplexing/de-multiplexing in FIG. 1(b) can filter out the out-band noise, but cannot filter out the in-band noise. After transmission through different links, the channels may have different OSNRs, and the in-band OSNR can be quite different from the out-band OSNR.
Thus, there are proposed many techniques for monitoring in-band OSNR, which can be generally classified into polarization nulling methods, waveform sampling methods, signal spectral analysis methods, methods based on nonlinear Kerr effect and methods based on optical interferometry.
OSNR monitoring based on polarization nulling methods is a simple, cost-saving and effective monitoring method for OSNRs and this method will not be limited by the transmission speed and the modulation format. However, it has a serious defect that in its assumptions the data signal of the signal to be measured is considered as completely polarized and the noise is considered as un-polarized, however in practice, owing to several uncertain factors in optical fiber in transmission link such as birefringence, polarization-dependent gains and polarization-dependent loss etc., the data signal can be “depolarized” or the noise can be partially polarized, in which case that the operator cannot tell the difference between the data signal and the noise, and then the accuracy of the measurement cannot be guaranteed which requires further improvement.
Waveform sampling method is another technique that analyzes the waveform of the signal to be measured and analyzes sampled data statistically to assess OSNRs of the signal to be measured, which actually belongs to the field of electrical monitoring but can also be considered as in the field of optical performance monitoring since the sampling part can be implemented in the optical field. Waveform sampling can be further classified into synchronous sampling, asynchronous sampling and delay-tap sampling. Among these methods, the synchronous sampling is a quite mature technique that can synchronously monitor the information of the signal such as SNR, time jitter, quality factor, quality of eye diagram, etc., but it requires clock extraction from the signal to be measured to maintain synchronicity between the sampling and the signal. Thus, it is difficult to be applied to the future high-speed network for exponential increase of cost when the signal has a higher rate. Asynchronous sampling is one of the most studied techniques which directly performs asynchronous sampling with a low frequency on the waveform of the signal and then analyzes the sampled data statistically to monitor the properties of the signal, and it is cost-saving and has no constraint on the rate of the signal and avoids clock extraction for the signal, but requires a lot of statistical analysis and calculation in the post-processing stage, and according to current documents, its feasibility is proved in theory, complete monitoring experiments for asynchronous sampling have not been performed. Delay tap sampling is a technique without need of clock extraction, which can monitor synchronously several damages such as chromatic dispersion, polarization mode dispersion and OSNR, but its greatest drawback is that the delay of the sampling for the signals on two parts are associated with the rates of the signals which requires a precise setting; i.e., the technique is not transparent to the rate of the signal and requires further study and improvement. In general, waveform sampling method is a statistical analysis for waveform of the signal and thus is relatively more effective on monitoring for amplitude modulated signals. Currently, it is still rare to see the waveform sampling method for monitoring the phase modulated signals, so the applicable modulation formats related therewith are limited.
Signal spectral analysis method is another technique which monitors the high-speed signals by smartly monitoring the low-speed signals, which avoids the use of the high-speed devices and lowers the cost, but it has certain disadvantages:
(i) due to the overlap of signal spectrum between the low-frequency RF (radio frequency) signal and the Wavelength-Division-Multiplexing data signal, the low frequency signal may interfere with the data signal, and the two affect each other, and the low frequency signal has a stringent requirement for its power which must be large enough to distinguish the data signal from the noise but cannot be too large to affect the transmission and reception of the data signal;
(ii) the complexity of the system is increased since the low frequency signal is loaded onto the channel.
Methods based on a nonlinear Kerr effect include monitoring the OSNRs utilizing nonlinear effects such as four-wave mixing effect, cross phase modulation effect, parametric amplification, and two photon absorption in nonlinear devices such as semiconductor optical amplifier, highly nonlinear optical fiber and other nonlinear waveguide devices.
Methods based on a nonlinear Kerr effect have typical advantages such as all optical operation and high transmission rate, but also have drawbacks. Due to the adoption of the nonlinear effects, it has a higher power requirement for the signal to be measured and even a precise alignment for the phase of the signal to be measured, and thus its application is limited.
Methods based on interferometry can tell the data signal from the noise based on the different coherence characteristics of the data signal and the noise (the data signal has coherence while the noise has no coherence or very poor coherence). Since the coherence characteristics of the data signal and the noise cannot be affected by the factors such as chromatic dispersion, polarization mode dispersion and the degree of polarization of the noise, the OSNR monitoring based on interferometry can be resistant to other damages, and thus it is an effective and reliable OSNR technique which becomes one of the most promising ones among the current monitoring techniques.
The typical structure for OSNR monitoring based on interferometry is a Mach-Zehnder interferometer as shown in FIG. 2, which is comprised of two 3 dB couplers and two interference arms. Since this technique needs to obtain the autocorrelation function of the data signal, which requires calibration on each of the optical transmitters. It usually requires turning off the noise in the channel, which is practically impossible.
This problem can be solved by a pair of Michelson interferometers having different time delays as shown in FIG. 3, and as described in E. Flood et al., “In-band OSNR monitoring using a pair of Michelson fiber interferometers”, Optics Express, Vol. 18, 2010, No. 4, pp. 3618-3624, which is incorporated herein by reference its entirety. However, the solution of Michelson interferometers has a poor stability due to its huge structure, which is all fiber, and has limitations such as inability to integrate, easiness to be influenced by environment and high power of input optical light.